1. Field
The present invention relates to data communication, and more particularly to techniques for processing data sporadically transmitted (if at all) at designated times on a code channel, such as paging indicator bits on a quick paging channel.
2. Background
A terminal in a wireless (e.g., cellular) communication system is typically designed to operate in one of several modes, such as active and standby, at any given moment. In the active mode, the terminal can actively exchange data with one or more base stations (e.g., for a voice or data call). And in the standby mode (which is also referred to as the idle mode), the terminal typically monitors a paging channel for messages addressed to the terminal. Such messages may include those for alerting the terminal to the presence of an incoming call (i.e., page messages) and those for updating system parameters for the terminal (i.e., overhead messages).
During the idle mode, a terminal continues to consume power to sustain the circuitry needed to monitor the signals transmitted from the base stations. Many terminals (e.g., cellular telephones) are portable and powered by an internal battery. The power consumption by the terminal in the idle mode decreases the available battery resources, which then shortens the xe2x80x9cstandbyxe2x80x9d time between battery recharges and the xe2x80x9ctalkxe2x80x9d time when a call is placed or received. Therefore, it is highly desirable to minimize the terminal""s power consumption in the idle mode to increase battery life.
In one technique for reducing power consumption in the idle mode, messages on the paging channel are sent to a terminal (if at all) at designated times. For IS-95 and cdma2000 systems, the paging channel is divided into numbered xe2x80x9cslotsxe2x80x9d, and the terminal may be assigned one or more slots by the base stations with which it has previously established communication. In such a slotted paging channel, the terminal periodically rather than continuously monitors the paging channel for messages from the base stations. The terminal wakes up from an xe2x80x9cinactivexe2x80x9d state prior to its assigned slot, enters an xe2x80x9cactivexe2x80x9d state and processes the paging channel for messages, and reverts back to the inactive state if additional communication is not required. The terminal remains in the active state (which is also referred to as the xe2x80x9cawakexe2x80x9d state) if a received message requires the terminal to perform additional actions. In the time period between successive presences in the active state, the terminal is in the inactive state and the base stations do not send any messages to the terminal.
In another technique for further reducing power consumption in the idle mode, a quick paging channel (QPCH) is used to indicate whether or not a page message may be transmitted on the paging channel for a terminal. The quick paging channel includes a number of paging indicator bits that are transmitted as binary On/Off bits. Each terminal is assigned two paging indicator bits for each (80 msec) QPCH slot, with the positions of the assigned paging indicator bits being determined based on a hashing function. The paging indicator bits may be more quickly detected, and if these bits indicate that no message will be transmitted on the paging channel for the terminal, then the terminal would not need to process the paging channel and may enter sleep.
In the inactive state, as much circuitry as possible is typically powered down to conserve power. This may entail removing power to certain analog circuitry (e.g., RF circuits) and gating off the clocks to certain digital circuitry. During a sleep, only a precision oscillator, a sleep timer, and some other necessary circuitry may be kept active.
To process the paging channel in the active state, the terminal needs to acquire and synchronize to the timing of the transmitting base stations. During re-acquisition in the initial part of the active state, the terminal typically searches for strong signal instances (or multipaths) in the received signal and acquires the timing and frequency of each found multipath of sufficient strength. The timing is typically obtained from the phase of a (complex) pseudo-random number (PN) sequence used to spread the data at the base station.
A full search of the entire PN code space for strong multipaths typically requires a long period of time, but would be required if the terminal has no knowledge of the timing of the multipaths when it wakes up from a sleep. To obviate the need for a full search, the digital circuitry is conventionally gated off for a precise duration of time such that the circuitry""s timing is approximately aligned with the system timing when the clocks are subsequently gated back on at wakeup.
For IS-95 and cdma2000 systems, a data frame is interleaved over 20 msec and the PN sequence has a duration of 26.67 msec. The shortest time period that is common to both the (20 msec) frame timing and the (26.67 msec) PN timing is 80 msec, which covers four frames and three PN sequences. If the sleep duration is selected to be an integer multiple of 80 msec, then the terminal""s timing will be approximately aligned with the system timing when the terminal wakes up from sleep, and only some integer numbers of frames and PN sequences will have elapsed since the terminal enters sleep. With approximately correct timing at wakeup, only a limited search needs to be performed to find and acquire the multipaths. Thus, the sleep duration is conventionally selected to be an integer multiple of 80 msec, or the least common multiple of the frame and PN timing.
A coarse increment of 80 msec for the sleep duration limits the terminal""s ability to quickly transition into and out of sleep. Consequently, the terminal spends more time in the active state then necessary to process the required channel. Since power consumption in the active state is typically many times greater than that in the inactive state, any reduction in the amount of time spent in the active state may result in a direct and significant improvement in standby time.
There is therefore a need in the art for techniques to more efficiently process sporadically transmitted data (e.g., paging indicator bits on the quick paging channel) to reduce power consumption.
Aspects of the invention provide techniques to process data sporadically transmitted (if at all) at designated times in a wireless communication system, such as paging indicator (PI) bits on the quick paging channel (QPCH) and page messages on the paging channel (PCH) in cdma2000. The techniques described herein support a sleep cycle that may start at virtually anytime and having a sleep duration that may further be selected based on a fine time increment (or xe2x80x9csleep quantumxe2x80x9d). For example, the sleep quantum may be selected as 512 PN chips (which is 416.6 xcexcsec in cdma2000) for an example design described below.
The techniques described herein ensure that proper timing is maintained for the finger processors and the symbol combiner of a rake receiver typically used to demodulate a received signal in a CDMA system. In an aspect, the sleep quantum may be selected to be an integer multiple of the size of the symbol buffers used in the symbol combiner and an integer multiple of the PN phase shift achievable through xe2x80x9cmaskingxe2x80x9d. This sleep quantum ensures that the finger processors and symbol combiner may be easily moved to the proper positions upon wakeup from a sleep. Various techniques are also provided herein to align the symbol combiner timing to that of a particular multipath in the received signal, if necessary.
In one specific application, the techniques described herein may be advantageously used to detect PI bits transmitted on the QPCH. With the ability to select a sleep duration in relatively fine increments (e.g., integer multiples of 512 PN chips) and to start the sleep and to wake up at just about any times, the terminal may perform a sleep between a pair of assigned PI bits in the same QPCH slot, or between an assigned PI bit and the start of a PCH slot. With the ability to wake up and process a single PI bit, power consumption is reduced and standby time is extended.
The techniques described herein may be used for various CDMA and wireless communication systems, such as IS-95, cdma2000, and W-CDMA.
The invention further provides methods, apparatus (e.g., terminal), and other elements that implement various aspects, embodiments, and features of the invention, as described in further detail below.